1. Field of the Invention
The present invention relates to a humidity dependence improver for polymer film, a polymer film containing it, a protective film for polarizer, a retardation film, a polarizer containing the film and a liquid crystal display device, especially a VA (vertical aligned)-mode liquid crystal display device.
2. Description of the Related Art
The display characteristics of liquid crystal display devices are being improved more and more these days, and it is known that a retardation film having a specific Re and a specific Rth is placed between a polarizer and a liquid crystal cell to remarkably improve viewing angle characteristics of the liquid crystal display device.
As one of the methods for controlling Re and Rth of a retardation film, JP-A 2004-109410 discloses a method of adding a retardation enhancer to a polymer film. The retardation enhancer disclosed in this reference is a compound containing a keto-enol tautomerizable structure as its constitutive element and capable of forming a molecular complex; and as one example thereof, the reference discloses a compound having a guanamine skeleton such as 1,3,5-triazine ring. As other retardation enhancer, JP-A 2001-166144 and 2003-344655 disclose a discotic compound and a compound having a 1,3,5-triazine ring-containing structure other than the above.
On the other hand, the present inventors have investigated other characteristics of the polymer film containing such a retardation enhancer, and have found that Re and Rth of the film greatly fluctuate depending on the change in the humidity of the usage environment (that may be referred to as humidity dependence of Re and Rth).